I have done a couple of cruises before. I was on a ship out of Saebo, Japan. Stock-up on supplements such as a mulit-vitamin, protien powder and protien bars before deployment. I was on a LPD. At first the gym was useful, but was old. Just before I left the ship our dept head, whom was the MWR officer spent over 15,000 on the new gym. We got a squat rack, a few benches, adjustable benches and a few other goodies.
I did find it hard to train once in a while, especially going to Korea. The water near there is rough during the winter. About a week before you come back from deployement you will find people in the weight room, that you have never seen before, and once you get to your homeport, you will never see these people in the gym. We use to call these guys "weekend Arnolds". If you go to Austrila and Thailand you might be able to work out at their gyms for free.